The technique of immuno-electron microscopy has been used to follow the synthesis of tissue specific proteins of the chicken lens. These proteins are the alpha, beta, and delta crystallins. Antisera specific to these proteins are labelled with horse-radish peroxidase, and these enzyme-labelled antibody conjugates, because they retain both immunologic and enzymatic activities, can be used as cytochemical stains in the electron microscope. We have used this technique at the ultrastructural level to determine that the first crystallin (delta) can be detected in the differentiating lens cells at 54 hrs. This result has been corroborated by both immunochemical and biochemical procedures. Some aspects of the immuno-electron microscopic study remain to be satisfied. We hope to devote our efforts at improving the penetrability of the peroxidase-labelled antibody into the cells by using Fab fragments instead of gamma globulin. Another study projected for the coming year is to explore the usefulness of a new fixative for immuno- electron microscopy. This fixative contains periodate, lysine, and paraformaldehyde, and has been reported to be useful for the study of structures rich in ribosomes and glycogen. Finally, by using the successive staining and elution procedure whereby a section is stained with a labelled antiserum and then eluted, we hope to obtain an answer to the question of whether the same cells have the capacity for synthesizing more than one crystallin. Bibliographic references: T. Shinohara and A. Katoh, 1975, Delta crystallin synthesis during chick lens differentiation. II. Expression of programmed potential in vitro, Experimental Eye Research 20:341-347.